Lithographic proximity correction (LPC) is used in VLSI circuits to correct for printing errors such as two-dimensional lithographic errors. In integrated circuits having printed shapes with sizes that approach the wavelength of exposure, two-dimensional lithographic printing errors may occur because of electromagnetic diffraction from orthogonal edges of a mask feature during the lithographic printing process. Additionally, photoresist diffusion and etch effects can cause two dimensional lithographic printing errors. LPC is used to correct for these optical and non-optical undesirable printing errors by adding or subtracting other features or shapes to or from the original design shapes. These other features are used to modify the original design in order to compensate for certain undesirable effects that may take place during printing.
FIGS. 1 and 2 together are used to illustrate the use of LPC shapes in the prior art. Specifically, FIG. 1 illustrates a portion of a semiconductor design database 100 having multiple shapes. Shapes 110 and 115 would generally represent an interconnect structure, such as a metal-1 interconnect for a semiconductor device. The shapes 120-123 in FIG. 1 represent contacts. Contacts are used to connect different layers within a semiconductor design. In other words, the metal-1 interconnect 110 would be connected to another interconnect layer, which is not shown, through the contacts 120 and 122. The shape 124 would represent a via, whereby a via connects a specific interconnect layer to another interconnect layer (e.g. metal-2). As the shapes 110 and 115 become narrower in size as technologies improve, the lithographic printing errors can become increasingly significant. For example, narrow line ends tend to foreshorten. Therefore, as illustrated in FIG. 2, the use of LPC shapes 219, 220, 221, 223, 224, 225, 231, 230, 226, 227, 228, and 229, have been added in the prior art in order to allow proper lithographic resolution of the shapes 110 and 115.
While the additions of the assist features illustrated in FIG. 2 allow for more precise replication of the shapes 110 and 115 as described in the data base, other problems are introduced. A first problem associated with the adding of the LPC features illustrated in FIG. 2, is that the size of the database file greatly increases. For the shape 110, where previously a shape having eight vertices needed to be stored, following the LPC additions, a total of 32 vertices must be stored in order to represent the shape with the LPC shapes added. Therefore, the amount of database room needed to store a prior art LPC design is significantly greater than the initial design itself. In addition, the amount of time needed to identify and add the LPC shapes can be significant. However, a yet more important issue with the prior art use of LPC features is that an interaction between adjacent LPC features can occur causing errors in the over all database.
One problem with interact ion between the system features can be best understood with reference to features 226 and 228 of FIG. 2. Specifically, if following the addition of assist feature 228 to interconnect 115, and the addition of LPC feature 226 to shape 110, it is possible that features 226 and 228 may now reside in close enough proximity to each another such th at an electrical short may be created during the manufacturing process. Such a short would most likely cause a failure in the overall semiconductor device 100.
Therefore, the basic assumption made when LPC features are added, is that the LPC feature allows the proper resolution of the desired shape, such as shape 115, and that the LPC features themselves do not appear on this final manufactured device. However, when LPC features are placed in such close proximity, such as features 226 and 228, it is possible for the shapes 115 and 110 to be modified such that the LPC features themselves are now resolved onto the final semiconductor device there by causing problems.
Therefore, it would be desirable for an LPC process to be capable of eliminating the problems associated with LPC features in close proximity.